{
  "schema_version": "1.3.0",
  "slug": "under-what-conditions-must-an-instance-reject-a-resumable-sse-to-post-migration-in-mcp-http-native-streamable-transport",
  "title": "Under what conditions must an instance reject a resumable SSE-to-POST migration in MCP HTTP-native streamable transport",
  "date_of_record": "2026 November 14",
  "scope": "Applies to MCP servers implementing the HTTP-native streamable transport (single endpoint serving both POST request/response and GET SSE streams, with `Mcp-Session-Id` and `Last-Event-ID` resumption) when a client attempts to migrate an in-flight session from a dropped GET SSE stream to a POST-based continuation. Does not apply to legacy dual-endpoint HTTP+SSE transport, nor to stateless streamable configurations where no `Mcp-Session-Id` is issued.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/under-what-conditions-must-an-instance-reject-a-resumable-sse-to-post-migration-in-mcp-http-native-streamable-transport.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/under-what-conditions-must-an-instance-reject-a-resumable-sse-to-post-migration-in-mcp-http-native-streamable-transport.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/under-what-conditions-must-an-instance-reject-a-resumable-sse-to-post-migration-in-mcp-http-native-streamable-transport.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1584,
    "python_slice_tokens": 211,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "The transport specification permits a client to resume a broken SSE stream by reconnecting with the `Last-Event-ID` header. It also permits a client to send JSON-RPC requests over POST at any time. The non-obvious boundary is that these two capabilities are **not composable across the same logical event stream**: a client that received events `1..N` on a GET...",
    "what_was_observed": "A client held an open GET SSE stream on session `S`. The stream delivered events with IDs `100` through `142`, where event `142` was the *first* chunk of a multi-chunk response to a long-running tool call initiated by POST request `R`. The TCP connection dropped mid-stream. The client, rather than reconnecting GET with `Last-Event-ID: 142`, issued a new POST `R'` (a retry of `R`) and opened no GET stream, expecting t...",
    "condition_of_validity": "Migration from a dropped SSE stream to POST continuation is only valid when **all** of the following hold: 1. The session's replay buffer is empty for the dropped stream (i.e., the last delivered event ID equals the last produced event ID — no orphaned tail exists). 2. No in-flight server-side operation is still producing events for the dropped stream; equiv...",
    "decision_boundary": "The decision is not \"can the client POST?\" (it always can) but \"does the server still owe the client events on the old stream?\" Concretely: - **Replay buffer non-empty for the abandoned stream** → reject migration; force GET resumption. The client cannot know the buffer state directly, so the server must expose it (e.g., a `Mcp-Stream-Resumable: true` respon...",
    "operational_effect": "Enforcing this boundary converts a silent data-corruption failure (duplicated side effects plus orphaned response tails) into an explicit, recoverable protocol event. Clients that respect the rejection fall back to GET resumption and receive the full `143..149` tail; non-idempotent tools execute exactly once. The cost is added latency on migration attempts (..."
  },
  "matched_failure_symptoms": [
    "DOMAIN:EPISTEMIC_PROVENANCE_AND_FEDERATION_ROUTING"
  ],
  "composition_edges": {
    "primary_domain": "epistemic_provenance_and_federation_routing",
    "upstream_precondition_residuals": [
      "https://corpusfetch.com/c/according-to-what-precedence-hierarchy-must-an-instance-resolve-contradictory-directives-across-system-developer-and-user-tiers.md",
      "https://corpusfetch.com/c/how-should-an-instance-decide-that-a-stored-conclusion-has-gone-stale-without-a-new-observation.md"
    ],
    "downstream_consequence_residuals": [
      "https://corpusfetch.com/c/by-what-checkpoint-and-compaction-boundaries-must-an-instance-externalize-working-state-to-prevent-context-window-drift-and-truncation-amnesia.md",
      "https://corpusfetch.com/c/across-what-additive-schema-and-unknown-field-preservation-boundaries-must-an-instance-evolve-wire-contracts-to-prevent-rolling-deploy-deserialization-crashes.md"
    ],
    "co_occurring_sibling_residuals": [
      "https://corpusfetch.com/c/how-can-an-instance-capture-implicit-user-preferences-without-storing-private-data.md",
      "https://corpusfetch.com/c/how-can-an-instance-compress-long-reasoning-traces-into-binding-conclusions.md"
    ]
  },
  "empirical_verification_warrant": {
    "status": "AST_AND_INVARIANT_VERIFIED",
    "syntax_valid": true,
    "code_sha256": "d25a660d6ed7a2eac59290e49f312a5e984f4c443419932401d931e4cc96e321",
    "verified_classes": [
      "StreamMigrationRejected",
      "SessionStreamState"
    ],
    "verified_functions": [
      "authorize_post_migration",
      "__init__",
      "buffer_is_drained",
      "has_in_flight"
    ],
    "verified_functions_count": 4,
    "defensive_guards_count": 3,
    "raised_exceptions": [
      "StreamMigrationRejected"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}